Microporous Polymeric Membrane, Battery Separator, and Battery

ABSTRACT

The invention relates to microporous polymeric membrane having a good balance of rupture temperature and air permeability. The invention also relates to a battery separator formed by such a microporous membrane, and a battery comprising such a separator. Another aspect of the invention relates to a method for making the microporous polymeric membrane, a method for making a battery using such a membrane as a separator, and a method for using such a battery.

FIELD OF THE INVENTION

The invention relates to a microporous membrane having suitable permeability, pin puncture strength, shutdown temperature, and rupture temperature. The invention also relates to a battery separator formed by such multi-layer, microporous membrane, and a battery comprising such a separator. Another aspect of the invention relates to a method for making the multi-layer, microporous polyolefin membrane, a method for making a battery using such a membrane as a separator, and a method for using such a battery.

BACKGROUND OF THE INVENTION

Microporous membranes can be used as battery separators in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous polyolefin membranes are used for battery separators, particularly lithium ion battery separators, the membranes' performance significantly affects the properties, productivity and safety of the batteries. Accordingly, the microporous polyolefin membrane should have suitable mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc. As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown (or rupture) temperature for improved battery-safety properties, particularly for batteries that are exposed to high temperatures during manufacturing, charging, re-charging, use, and/or storage. Improving separator permeability generally leads to an improvement in the battery's storage capacity. High shutdown speed is desired for improved battery safety, particularly when the battery is operated under overcharge conditions. Improving pin puncture strength is desired because roughness of the battery's electrode can puncture the separator during manufacturing leading to a short circuit. Improved thickness uniformity is desired because thickness variations lead to manufacturing difficulties when winding the film on a core.

In general, microporous membranes containing polyethylene only (i.e., the membrane consists of, or consists essentially of, polyethylene) have low meltdown temperatures, while microporous membranes containing polypropylene only have high shutdown temperatures. Accordingly, microporous membranes comprising polyethylene and polypropylene as main components have been proposed as improved battery separators. It is therefore desired to provide microporous membranes formed from polyethylene resin and polypropylene resin, and multi-layer, microporous membranes comprising polyethylene and polypropylene.

JP7-216118A, for example, discloses a battery separator comprising a multi-layer, porous film having two microporous layers. Both layers can contain polyethylene and polypropylene, but in different relative amounts. For example, the percentage of the polyethylene is 0 wt. % to 20 wt. % in the first microporous layer, and 21 wt. % to 60 wt. % in the second microporous layer, based on the combined weight of the polyethylene and polypropylene. The total amount of polyethylene in the film (i.e., both microporous layers) is 2 wt. % to 40 wt. %, based on the weight of the multi-layer microporous film.

JP10-195215A discloses a relatively thin battery separator having conventional shutdown and pin-pulling characteristics. The term “pin pulling” refers to the relative ease of pulling a metal pin from a laminate of a separator, a cathode sheet and an anode sheet, which is wound around the pin, to form a toroidal laminate. The multi-layer, porous film contains polyethylene and polypropylene, but in different relative amounts. The percentage of polyethylene is 0 wt. % to 20 wt. % in the inner layer and 61 wt. % to 100 wt. % in the outer layer, based on the total weight of the polyethylene and polypropylene.

JP10-279718A discloses a separator designed to prevent unacceptably large temperature increases in a lithium battery when the battery is overcharged. The separator is formed from a multi-layer, porous film made of polyethylene and polypropylene, with different relative amounts of polyethylene and polypropylene in each layer. The film has a polyethylene-poor layer whose polyethylene content is 0 wt. % to 20 wt. %, based on the weight of the polyethylene-poor layer. The second layer is a polyethylene-rich layer which contains 0.5 wt. % or more of polyethylene having a melt index of 3 or more and has a polyethylene content of 61 wt. % to 100 wt. %, based on the weight of the polyethylene-rich layer.

It would be desirable to further improve the permeability and pin puncture strength of microporous membranes.

SUMMARY OF THE INVENTION

In an embodiment, the invention relates to a multi-layer microporous membrane, comprising:

a first layer material comprising polyethylene and a second layer material comprising polypropylene, the polypropylene having (1) a weight-average molecular weight of 6×10⁵ or more, (2) a heat of fusion of 90 J/g or more, and (3) a molecular weight distribution (“MWD” defined as Mw/Mn) in the range of about 2 to about 6, wherein

(a) the multi-layer microporous membrane has at least a first microporous layer containing the first layer material, a third microporous layer containing the first layer material, and a second microporous layer containing the second layer material, the second microporous layer being located between the first and third microporous layers, and

(b) the total amount of polypropylene in the multi-layer microporous membrane is at least 2.0 wt. % based on the total weight of the microporous membrane.

In another embodiment, the invention relates to a method for producing a microporous membrane, comprising,

-   (1) combining a polyethylene resin and a first diluent, -   (2) combining a polypropylene resin, and a second diluent; the     polypropylene resin having the polypropylene having (a) a     weight-average molecular weight of 6×10⁵ or more, (b) a heat of     fusion of 90 J/g or more, and (c) an MWD in the range of 2 to 6; -   (3) extruding at least a portion of the combined first polyethylene     resin and first diluent, and extruding at least a portion of the     combined first polypropylene resin and second diluent to produce a     multi-layer extrudate which comprises     -   a first layer comprising the combined polyethylene and first         diluent, a second layer comprising the polypropylene and the         second diluent, and a third layer comprising the combined         polyethylene and first diluent, wherein the second layer is         located between the first and third layers and wherein the         polypropylene is present in the extrudate in an amount ≧2.0 wt.         % based on the weight of polymer in the extrudate; and then -   (4) cooling the multi-layer extrudate to form a multi-layer sheet, -   (5) removing at least a portion of the first and second diluents     from the multi-layer sheet to form a diluent-removed sheet, and -   (6) removing at least a portion of any volatile species from the     sheet to form the microporous membrane.

In yet another embodiment, the invention relates to a microporous polymeric membrane having a rupture temperature of 180° C. or higher and an air permeability satisfying the relationship A≦0.097 P-I where A is the microporous membrane's Normalized Air Permeability expressed in the units of sec/100 cm³/25 μm, P is the microporous membrane's Pin Puncture Strength expressed in the units of mN/25 μm, and I is in the range of from 100 to about 250, or about 110 to about 230. In an embodiment, I is about 110, in which case the relationship can be expressed as A≦0.097 P-110.

In yet another embodiment, the invention relates to a microporous membrane having Rupture temperature 180° C. or higher and comprising polypropylene wherein the total amount polypropylene in the membrane is ≧2 wt. %, based on the total weight of the membrane. The invention is not limited to microporous membranes. For example, separators comprising such membranes, batteries comprising such separators, and the use of such batteries are all within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing Air permeability (Y axis) and Pin Puncture Strength (X axis) for various microporous membranes. The membranes described in the Examples are represented by diamonds and the membranes described in the Comparative Examples are represented by triangles. Membranes of the invention but not further exemplified are represented by rectangles. Points represented by circles are membranes which have (i) a Rupture temperature lower (cooler) than 180° C. and/or (ii) an Air Permeability that is greater than 0.097P-I, where I is in the range of about 100 to about 250. I represents the Y intercept of the line plotted on FIG. 1, which has a slope of about 0.097. Air Permeability is expressed in the units of sec/100 cm³/25 μm and P is the microporous membrane's Pin Puncture Strength expressed in the units of mN/25 μm (where “/25 μm” means normalized to the value for a membrane of 25 μm thickness).

FIG. 2 is a graph showing the Rupture temperature of microporous polymeric membranes as a function of the membrane's total polypropylene content, e.g., the total amount as measured by wt. % of first and second polypropylene in the membrane, based on the total weight of the membrane. The membranes described in the Examples are represented by diamonds and the membranes described in the Comparative Examples are represented by triangles. Membranes not further exemplified are represented by asterisks.

DETAILED DESCRIPTION OF THE INVENTION [1] Composition and Structure of the Microporous Membrane

In an embodiment, the multi-layer, microporous membrane comprises three layers, wherein the outer layers (also called the “surface” or “skin” layers) comprise the first microporous layer material and at least one intermediate layer (or “core” layer) comprises the second microporous layer material. In a related embodiment, the multi-layer, microporous membrane can comprise additional layers, i.e., in addition to the two skin layers and the core layer. In a related embodiment where the multi-layer, microporous membrane comprises three or more layers, the outer layers consist essentially of (or consists of) the first microporous layer material and at least one intermediate layer consists essentially of (or consists of) the second microporous layer material. While it is not required, the core layer can be in planar contact with one or more of the skin layers in a stacked arrangement such as A/B/A with face-to-face stacking of the layers. The membrane can be referred to as a “polyolefin membrane” when the membrane contains polyolefin. While the membrane can contain polyolefin only, this is not required, and it is within the scope of the invention for the membrane to contain polyolefin and materials that are not polyolefin.

When the multi-layer, microporous membrane has three or more layers, the multi-layer, microporous polyolefin membrane has at least one layer comprising the first microporous layer material and at least one layer comprising the second microporous layer material.

In an embodiment, the microporous membrane is a three layer membrane wherein the thickness of the core layer is in the range of about 4.6% to about 50%, or from about 5% to about 30%, or from 5% to about 15% of the total thickness of the multi-layer microporous membrane.

In an embodiment, the first microporous layer material comprises a first polyethylene and optionally a first polypropylene. The second microporous layer material comprises a second polypropylene and optionally a second polyethylene. The total amount of polyethylene in the multi-layer, microporous polyolefin membrane is in the range of from about 70 wt. % to about 98 wt. %, or from about 90 wt. % to about 97.95 wt. %, or from about 95 wt. % to about 97.9 wt. %, based on the weight of the multi-layer, microporous membrane. The total amount of polypropylene in the multi-layer, microporous membrane is generally greater than 2.0 wt. % based on the total weight of the membrane. When the membrane contains material in addition to polyolefin, the wt. % is based on the weight of the membrane's total polyolefin content. For example, the total amount of polypropylene in the multi-layer, microporous membrane can be in the range of from about 2.0 wt. % to about 30 wt. %, e.g., about 2.05 wt. % to about 10 wt. %, or from about 2.1 wt. % to about 5 wt. %, based on the weight of the multi-layer, microporous membrane. In an embodiment, the first polyethylene is present in the first microporous layer material in a first polyethylene amount in the range of from about 80 wt. % to about 100 wt. % based on the weight of the first microporous layer material; the first polypropylene is present in the first microporous layer material in a first polypropylene amount in the range of from about 0 wt. % to about 10 wt. % (e.g., from about 0.5 wt. % to about 10 wt. %) based on the weight of the first microporous layer material; the second polyethylene is present in the second microporous layer material in a second polyethylene amount in the range of from about 0 wt. % to about 99 wt. %, e.g., from about 40 wt. % to about 90 wt. %, such as from about 50 wt. % to about 80 wt. % based on the weight of the second microporous layer material; and the second polypropylene is present in the second microporous layer material in a second polypropylene amount in the range of from about 1 wt. % to about 100 wt%, e.g., from about 10 wt. % to about 60 wt. %, such as from about 20 wt. % to about 50 wt. % based on the weight of the second microporous layer material.

The first and second polyethylene and the first and second polypropylene will now be described in more detail.

A. The First Polyethylene

In an embodiment, the first polyethylene is a polyethylene having an Mw in the range of about 1×10⁴ to about 1×10⁷, or about 1×10⁵ to about 5×10⁶, or about 1×10⁵ to about 9×10⁵. Although it is not critical, the first polyethylene can have terminal unsaturation of, e.g., two or more per 10,000 carbon atoms in the polyethylene. Terminal unsaturation can be measured by, e.g., conventional infrared spectroscopic methods. The first polyethylene can be one or more varieties of polyethylene, e.g., PE1, PE2, etc. In one embodiment, the first polyethylene comprises PE1. PE1 comprises polyethylene having an Mw ranging from about 4×10⁵ to about 8×10⁵. Optionally, the PE1 can be one or more of an high density polyethylene (“HDPE”), a medium-density polyethylene, a branched low-density polyethylene, or a linear low-density polyethylene. In an embodiment, PE1 has an Mn/Mw of ≦100, e.g., in the range of about 3 to about 20. In an embodiment, PE1 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer such as propylene, butene-1, hexene-1, etc, typically in a relatively small amount compared to the amount of ethylene, e.g., 10 mol % or less based on 100% by mol of the copolymer. Such a copolymer can be produced using a single-site catalyst.

In an embodiment, the first polyethylene comprises PE2. PE2 comprises polyethylene having an Mw of at least about 1×10⁶. For example, PE2 can be an ultra-high molecular weight polyethylene (“UHMWPE”). In an embodiment, PE2 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer which is typically present in a relatively small amount compared to the amount of ethylene, e.g., 10 mol % or less based on 100% by mol of the copolymer. The comonomer can be, for example, one or more of propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, or styrene. Optionally, the Mw of PE2 can be in the range, e.g., of from about 1×10⁶ to about 15×10⁶, or from about 1×10⁶ to about 5×10⁶, or from 1×10⁶ to about 3×10⁶. Optionally, the PE2 has an Mw/Mn in the range of about 4.5 to about 10.

In an embodiment, the first polyethylene comprises both PE1 and PE2. In this case, the amount of PE2 in the first polyethylene can be, e.g., in the range of about 0 wt % to about 50 wt %, or about 1 wt. % to about 50 wt. %, based on the weight of the first polyethylene. In one embodiment, the first polyethylene comprises PE1 in an amount ≧92 wt. % and PE2 in an amount ≦8 wt. %, based on the weight of the first polyethylene.

In one embodiment, the first polyethylene has one or more of the following independently-selected features:

-   (1) The first polyethylene comprises PE1. -   (2) The first polyethylene consists essentially of, or consists of,     PE1. -   (3) The PE1 is one or more of a high-density polyethylene, a     medium-density polyethylene, a branched low-density polyethylene, or     a linear low-density polyethylene. -   (4) PE1 is one or more of a high-density polyethylene having an Mw     ranging from about 1×10⁵ to about 5×10⁶, e.g., from about 2×10⁵ to     about 9×10⁵, such as from about 4×10⁵ to about 8×10⁵. -   (5) PE1 is at least one of (i) an ethylene homopolymer or (ii) a     copolymer of ethylene and a third α-olefin selected from the group     of propylene, butene-1, hexene-1. -   (6) The first polyethylene comprises both PE1 and PE2. -   (7) PE2 has an Mw ranging from about 1×10⁶ to about 15×10⁶, e.g.,     from about 1×10⁶ to about 5×10⁶, such as from about 1×10⁶ to about     3×10⁶, and an Mw/Mn in the range of about 4.5 to about 10. -   (8) PE2 is ultra-high-molecular-weight polyethylene. -   (9) PE2 is at least one of (i) an ethylene homopolymer or (ii) a     copolymer of ethylene and a fourth α-olefin selected from the group     of propylene, butene-1, hexene-1. -   (10) The first polyethylene has a molecular weight distribution     (“Mw/Mn”) of about 5 to about 300, or about 5 to about 100, or     optionally from about 5 to about 30.

In an embodiment, the microporous membrane is characterized by one or more of:

-   (1) the amount of the PE1 in the first microporous layer material is     in the range of from about 50 wt. % to about 100 wt. %, based on the     weight of the first microporous layer material; -   (2) the amount of the PE2 in the first microporous layer material is     in the range of from about 0 wt. % to about 50 wt. %, based on the     weight of the first microporous layer material; -   (3) the amount of the PE1 in the second microporous layer material     is in the range of from about 40 wt. % to about 60 wt. %, based on     the weight of the second microporous layer material; or -   (4) the amount of the PE2 in the second microporous layer material     is in the range of from about 0 wt. % to about 50 wt. %, based on     the weight of the second microporous layer material.

B. The Second Polyethylene

The second polyethylene can be selected from among the same polyethylenes as the first polyethylene. For example, the second polyethylene comprise PE1, PE2, or both PE1 and PE2. When the second polyethylene comprises PE1 and PE2, the amount of PE2 in the second polyethylene can be in the range of 0 wt % to about 50 wt. %, or about 1 wt. % to about 50 wt. %, based on the weight of the second polyethylene. Optionally, the second polyethylene is substantially the same as the first polyethylene.

Mw and MWD of the polyethylene and polypropylene are determined using a High Temperature Size Exclusion Chromatograph, or “SEC”, (GPC PL 220, Polymer Laboratories), equipped with a differential refractive index detector (DRI). The measurement is made in accordance with the procedure disclosed in “Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (2001)”. Three PLgel Mixed-B columns available from (available from Polymer Laboratories) are used for the Mw and MWD determination. For polyethylene, the nominal flow rate is 0.5 cm³/min; the nominal injection volume is 300 μL; and the transfer lines, columns, and the DRI detector are contained in an oven maintained at 145° C. For polypropylene, the nominal flow rate is 1.0 cm³/min; the nominal injection volume is 300 μL; and the transfer lines, columns, and the DRI detector are contained in an oven maintained at 160° C.

The GPC solvent used is filtered Aldrich reagent grade 1,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm of butylated hydroxy toluene (BHT). The TCB was degassed with an online degasser prior to introduction into the SEC. The same solvent is used as the SEC. Polymer solutions were prepared by placing dry polymer in a glass container, adding the desired amount of the TCB solvent, and then heating the mixture at 160° C. with continuous agitation for about 2 hours. The concentration of UHMWPE solution was 0.25 to 0.75 mg/ml. Sample solution are filtered off-line before injecting to GPC with 2 μm filter using a model SP260 Sample Prep Station (available from Polymer Laboratories).

The separation efficiency of the column set is calibrated with a calibration curve generated using a seventeen individual polystyrene standards ranging in Mp (“Mp” being defined as the peak in Mw) from about 580 to about 10,000,000. The polystyrene standards are obtained from Polymer Laboratories (Amherst, Mass.). A calibration curve (logMp vs. retention volume) is generated by recording the retention volume at the peak in the DRI signal for each PS standard and fitting this data set to a 2nd-order polynomial. Samples are analyzed using IGOR Pro, available from Wave Metrics, Inc.

C. The First Polypropylene

Besides polyethylene, the first layer materials can optionally comprise polypropylene. The polypropylene can be, for example, one or more of (i) a propylene homopolymer or (ii) a copolymer of propylene and a comonomer. The copolymer can be a random or block copolymer. The comonomer can be, e.g., one or more of α-olefins such as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene, etc.; and diolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amount of the fifth olefin in the copolymer is preferably in a range that does not adversely affect properties of the multi-layer microporous membrane such as heat resistance, compression resistance, heat shrinkage resistance, etc. For example, the amount of the comonomer is generally >10% by mol based on 100% by mol of the entire copolymer. Optionally, the polypropylene has one or more of the following properties: (i) the polypropylene has an Mw ranging from about 1×10⁴ to about 4×10⁶, or about 3×10⁵ to about 3×10⁶; (ii) the polypropylene has an Mw/Mn ranging from about 1.01 to about 100, or about 1.1 to about 50; (iii) the polypropylene's tacticity is isotactic; (iv) the polypropylene has a heat of fusion of at least about 90 Joules/gram; (v) polypropylene has a melting peak (second melt) of at least about 160° C., (vi) the polypropylene has a Trouton's ratio of at least about 15 when measured at a temperature of about 230° C. and a strain rate of 25 sec⁻¹; and/or (vii) the polypropylene has an elongational viscosity of at least about 50,000 Pa sec at a temperature of 230° C. and a strain rate of 25 sec⁻¹. Optionally, the first polypropylene is substantially the same as the second polypropylene.

D. The Second Polypropylene

In an embodiment, the second polypropylene has one or more of the following characteristics. The second polypropylene preferably has a weight-average molecular weight of 6×10⁵ or more (e.g., in the range of about 9×10⁵ to about 2×10⁶) and a heat of fusion ΔHm (measured by a differential scanning calorimeter (DSC) according to JIS K7122) of 90 J/g or more, and an Mw/Mn≦100, e.g., in the range of about 2 to about 6. Because polypropylene having a weight-average molecular weight of less than 6×10⁵ has low dispersibility in the polyethylene resin, its use makes stretching difficult, providing large micro-roughness to a surface of the second porous layer and large thickness variation to the multi-layer, microporous membrane. When the polypropylene has a heat of fusion ΔHm of less than 90 J/g, the resultant multi-layer, microporous membrane may have low meltdown properties and permeability.

The weight-average molecular weight of the second polypropylene can be, e.g., 8×10⁵ or more, or in the range 8×10⁵ to 2.0×10⁶. The heat of fusion ΔHm of the polypropylene is 95 J/g or more, or 100 J/g or more, or 110 J/g or more, or 115 J/g or more, e.g., in the range of 100 J/g to 120 J/g. The molecular weight distribution, Mw/Mn, is ≧2, e.g., in the range of 2 to 6. In an embodiment, Mw/Mn≧3, e.g, in the range of 3 to 10.

The polypropylene content of the second layer material can be, e.g., in the range of from about 1 wt. % to about 100 wt. %, e.g., from about 20 wt. % to about 80 wt. %, such as from about 20 wt. % to about 50 wt. %, based on the weight of the second layer material.

As long as the above conditions of the weight-average molecular weight, and the heat of fusion are met, the type of the polypropylene is not particularly critical, but may be a propylene homopolymer, a copolymer of propylene and a comonomer, or a mixture thereof, the homopolymer being preferable. The copolymer may be a random or block copolymer. The comonomer can be, for example, ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and combinations thereof The amount of comonomer is, e.g., less than 10 mol % based on 100 mol % of the copolymer. Optionally, the second polypropylene has one or more of the following properties: (i) the polypropylene has an Mw in the range of from about 1×10⁴ to about 4×10⁶, or about 6×10⁵ to about 3×10⁶; (ii) the polypropylene has an Mw/Mn in the range of from about 1.01 to about 100, or about 2 to about 6; (iii) the polypropylene's tacticity is isotactic; (iv) the polypropylene has a heat of fusion of at least about 95 Joules/gram; (v) the polypropylene has a melting peak “Tm” (second melt) of at least about 160° C., (vi) the polypropylene has a Trouton's ratio of at least about 15 when measured at a temperature of about 230° C. and a strain rate of 25 sec⁻¹; and/or (vii) the polypropylene has an elongational viscosity of at least about 50,000 Pa sec at a temperature of 230° C. and a strain rate of 25 sec⁻¹. In an embodiment, the polypropylene has a crystallinity ≧50%, e.g., in the range of 55% to about 75%, such as in the range of 65% to 70%.

The ΔHm, Mw, and Mn of polypropylene are determined by the methods disclosed in PCT Patent Publication No. WO2007/132942, which is incorporated by reference herein in its entirety.

[2] Materials Used to Produce the Multi-Layer, Microporous Polyolefin Membrane

A. Polymer Resins Used to make the First Microporous Layer Material

In an embodiment, the first microporous layer material is made by combining a first polyolefin composition and a first diluent to form, e.g., a first polyolefin solution. Since the diluent produces a multi-layer microporous membrane, the diluent can also be called a process solvent or a membrane-forming solvent. The resins used to make the first polyolefin composition will now be described in more detail.

(1) The First Polyethylene Resin

In an embodiment, the first polyethylene resin comprises the first polyethylene, where the first polyethylene is as described above in section [1]. For example, the first polyethylene resin can be a mixture of a polyethylene resin having a lower Mw than UHMWPE (such as HDPE) and UHMWPE resin.

Multi-stage polymerization can be used to obtain the desired Mw/Mn ratio in the first polyethylene resin. For example, a two-stage polymerization method can be used, forming a relatively high-molecular-weight polymer component in the first stage, and forming a relatively low-molecular-weight polymer component in the second stage. While not required, this method can be used, for example, when the first polyethylene resin comprises PE1. When the first polyethylene resin comprises the PE1 and PE2, the desired Mw/Mn ratio of the polyethylene resin can be selected by adjusting the relative molecular weights and relative amounts of the first and second polyethylene.

(2) The First Polypropylene Resin

Besides the first polyethylene resin, the first polyolefin composition can optionally further comprise a first polypropylene resin. In an embodiment, the first polypropylene resin comprises the first polypropylene, where the first polypropylene is as described above in section [1].

(3) Formulation

The amount of process solvent in the first polyolefin solution can be in the range. e.g., of from about 25 wt. % to about 99 wt. % based on the weight of the first polyolefin solution. In an embodiment, the amount of the first polyethylene resin in the first polyolefin composition can be in the range. e.g., of from about 50 wt. % to about 99 wt. % based on the weight of the first polyolefin composition. The balance of the first polyolefin composition can be the first polypropylene.

B. Polymer Resins Used to Produce the Second Microporous Layer Material

In an embodiment, the second microporous layer material is made from a second polyolefin solution that is generally selected independently of the first polyolefin solution. The second polyolefin solution comprises a second polyolefin composition and a second diluent which can be the same as the first diluent. As is the case in the first polyolefin solution, the second diluent can be referred to as a second membrane-forming solvent or a process solvent. In an embodiment, the second polyolefin composition comprises a second polyethylene resin and a second polypropylene resin. The second polyethylene resin comprises the second polyethylene as described above in section [1]. The second polypropylene resin comprises the second polypropylene as described above in section [1].

The amount of process solvent in the second polyolefin solution can be in the range. e.g., of from about 25 wt. % to about 99 wt. % based on the weight of the second polyolefin solution. In an embodiment, the amount of the second polyethylene resin in the second polyolefin composition can be in the range. e.g., of from about 5 wt. % to about 95 wt. % based on the weight of the second polyolefin composition. The balance of the second polyolefin composition can be the second polypropylene.

C. Third Polyolefin

Although it is not required, each of the first and second polyolefin compositions can further comprise a third polyolefin selected from the group consisting of polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene α-olefin copolymer (except for an ethylene-propylene copolymer). In an embodiment where a third polyolefin is used, the third polyolefin can, for example, have an Mw in the range of about 1×10⁴ to about 4×10⁶. In addition to or besides the third polyolefin, the first and/or second polyolefin composition can further comprise a polyethylene wax, e.g., one having an Mw in the range of about 1×10³ to about 1×10⁴. When used, these species should be present in amounts less than an amount that would cause deterioration in the desired properties (e.g., meltdown, shutdown, etc.) of the multi-layer, microporous membrane. When the third polyolefin is one or more of polybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, and polystyrene, the third polyolefin need not be a homopolymer, but may be a copolymer containing other α-olefins.

The multi-layer microporous membrane generally comprises the polyolefin used to form the polyolefin solution. A small amount of washing solvent and/or process solvent can also be present, generally in amounts less than 1 wt % based on the weight of the microporous polyolefin membrane. A small amount of polyolefin molecular weight degradation might occur during processing, but this is acceptable. In an embodiment, molecular weight degradation during processing, if any, causes the value of Mw/Mn of the polyolefin in the membrane to differ from the Mw/Mn of the first or second polyolefin solution by no more than about 50%, or no more than about 1%, or no more than about 0.1%.

[3] Production Method of Multi-Layer, Microporous Polyolefin Membrane

In an embodiment, the microporous polyolefin membrane is a two-layer membrane. In another embodiment, the microporous polyolefin membrane has at least three layers. For the sake of brevity, the production of the microporous polyolefin membrane will be mainly described in terms of two-layer and three-layer membranes, although those skilled in the art will recognize that the same techniques can be applied to the production of membranes or membranes having at least four layers.

In an embodiment, the three-layer microporous polyolefin membrane comprises first and third microporous layers constituting the outer layers of the microporous polyolefin membrane and a second layer situated between (and optionally in face-to-face contact with) the first and third layers. In an embodiment, the first and third layers are produced from a first mixture of polymer and diluent, e.g., a first polyolefin solution and the second (or inner) layer is produced from the second mixture of polymer and diluent, e.g., a second polyolefin solution. In another embodiment, the first and third layers are produced from the second polyolefin solution and the second layer is produced from the first polyolefin solution. While the invention is described in terms of extruding polyolefin solutions, it is not limited thereto, and any extrudable mixture of polymer and diluent can be used.

A. First Production Method

The first method for producing a multi-layer membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a first diluent to prepare a first polyolefin solution, (2) combining a second polyolefin composition and a second diluent to prepare a second polyolefin solution, (3) extruding (preferably simultaneously) the first and second polyolefin solutions through at least one die to form an extrudate, (4) cooling the extrudate to form a cooled extrudate, e.g., a multi-layer, gel-like sheet, (5) removing the membrane-forming solvent from the multi-layer, sheet to form a solvent-removed sheet, and (6) drying the solvent-removed gel-like sheet to remove volatile species, if any, in order to form the multi-layer, microporous polyolefin membrane. An optional stretching step (7), and an optional hot solvent treatment step (8), etc. can be conducted between steps (4) and (5), if desired. After step (6), an optional step (9) of stretching a multi-layer, microporous membrane, an optional heat treatment step (10), an optional cross-linking step with ionizing radiation (11), and an optional hydrophilic treatment step (12), etc., can be conducted if desired. The order of the optional steps is not critical.

(1) Preparation of First Polyolefin Solution

The first polyolefin composition comprises polyolefin resins as described above that can be combined, e.g., by dry mixing or melt blending with an appropriate membrane-forming solvent to produce the first polyolefin solution. Optionally, the first polyolefin solution can contain various additives such as one or more antioxidant, fine silicate powder (pore-forming material), etc., provided these are used in a concentration range that does not significantly degrade the desired properties of the multi-layer, microporous polyolefin membrane.

The first process solvent is optionally a solvent that is liquid at room temperature. While not wishing to be bound by any theory or model, it is believed that the use of a liquid solvent to form the first polyolefin solution makes it possible to conduct stretching of the gel-like sheet at a relatively high stretching magnification. In an embodiment, the first membrane-forming solvent can be at least one of aliphatic, alicyclic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, etc.; mineral oil distillates having boiling points comparable to those of the above hydrocarbons; and phthalates liquid at room temperature such as dibutyl phthalate, dioctyl phthalate, etc. In an embodiment where it is desired to obtain a multi-layer, gel-like sheet having a stable liquid solvent content, non-volatile liquid solvents such as liquid paraffin can be used, either alone or in combination with other solvents. Optionally, a solvent which is miscible with polyethylene in a melt blended state but solid at room temperature can be used, either alone or in combination with a liquid solvent. Such solid solvent can include, e.g., stearyl alcohol, ceryl alcohol, paraffin waxes, etc. Although it is not critical, it can be more difficult to evenly stretch the gel-like sheet or resulting membrane when the solution contains no liquid solvent.

The viscosity of the liquid solvent is not a critical parameter. For example, the viscosity of the liquid solvent can range from about 30 cSt to about 500 cSt, or from about 30 cSt to about 200 cSt, at 25° C. Although it is not a critical parameter, when the viscosity at 25° C. is less than about 30 cSt, it can be more difficult to prevent foaming the polyolefin solution, which can lead to difficulty in blending. On the other hand, when the viscosity is greater than about 500 cSt, it can be more difficult to remove the liquid solvent from the multi-layer microporous polyolefin membrane.

In an embodiment, the resins, etc., used to produce to the first polyolefin composition are dry mixed or melt-blended in, e.g., a double screw extruder or mixer. For example, a conventional extruder (or mixer or mixer-extruder) such as a double-screw extruder can be used to combine the resins, etc., to form the first polyolefin composition. The membrane-forming solvent can be added to the polyolefin composition (or alternatively to the resins used to produce the polyolefin composition) at any convenient point in the process. For example, in an embodiment where the first polyolefin composition and the first membrane-forming solvent are melt-blended, the solvent can be added to the polyolefin composition (or its components) at any of (i) before starting melt-blending, (ii) during melt blending of the first polyolefin composition, or (iii) after melt-blending, e.g., by supplying the first membrane-forming solvent to the melt-blended or partially melt-blended polyolefin composition in a second extruder or extruder zone located downstream of the extruder zone used to melt-blend the polyolefin composition.

When melt-blending is used, the melt-blending temperature is not critical. For example, the melt-blending temperature of the first polyolefin solution can range from about 10° C. higher than the melting point Tm₁ of the first polyethylene resin to about 120° C. higher than Tm₁. For brevity, such a range can be represented as Tm₁+10° C. to Tm₁+120° C. In an embodiment where the first polyethylene resin has a melting point of about 130° C. to about 140° C., the melt-blending temperature can be in the range of from about 140° C. to about 250° C., or from about 170° C. to about 240° C.

When an extruder such as a double-screw extruder is used for melt-blending, the screw parameters are not critical. For example, the screw can be characterized by a ratio L/D of the screw length L to the screw diameter D in the double-screw extruder, which can range, for example, from about 20 to about 100, or from about 35 to about 70. Although this parameter is not critical, when L/D is less than about 20, melt-blending can be more difficult, and when L/D is more than about 100, faster extruder speeds might be needed to prevent excessive residence time of the polyolefin solution in the double-screw extruder (which can lead to undesirable molecular weight degradation). Although it is not a critical parameter, the cylinder (or bore) of the double-screw extruder can have an inner diameter of in the range of about 40 mm to about 100 mm, for example.

The amount of the first polyolefin composition in the first polyolefin solution is not critical. In an embodiment, the amount of first polyolefin composition in the first polyolefin solution can range from about 1 wt. % to about 75 wt. %, based on the weight of the polyolefin solution, for example from about 20 wt. % to about 70 wt. %. Although the amount of first polyolefin composition in the first polyolefin solution is not critical, when the amount is less than about 1 wt. %, it can be more difficult to produce the multi-layer microporous polyolefin membrane at an acceptably efficient rate. Moreover, when the amount is less than 1 wt. %, it can be more difficult to prevent swelling or neck-in at the die exit during extrusion, which can make it more difficult to form and support the multi-layer, gel-like sheet, which is a precursor of the membrane formed during the manufacturing process. On the other hand, when the amount of first polyolefin composition solution is greater than about 75 wt. %, it can be more difficult to form the multi-layer, gel-like sheet. The amount of polymer (e.g., the first polyethylene resin) in the first polyolefin solution is preferably in the range of 1 wt. % to 50 wt. %, e.g., 20 wt. % to 40 wt. %, based on the weight of the first polyolefin solution. When the amount of polymer is less than 1 wt. %, swelling or neck-in may occur at the die exit during the extrusion of the first polyolefin solution to form a gel-like molding, resulting in decrease in the formability and self-support of the gel-like molding. On the other hand, when the amount of polymer is more than 50 wt. %, the formability of the gel-like molding is more difficult.

(2) Preparation of Second Polyolefin Solution

The second polyolefin solution can be prepared by the same methods used to prepare the first polyolefin solution. For example, the second polyolefin solution can be prepared by melt-blending a second polyolefin composition with a second membrane-forming solvent. The second membrane-forming solvent can be selected from among the same solvents as the first membrane-forming solvent. And while the second membrane-forming solvent can be (and generally is) selected independently of the first membrane-forming solvent, the second membrane-forming solvent can be the same as the first membrane-forming solvent, and can be used in the same relative concentration as the first membrane-forming solvent is used in the first polyolefin solution.

The second polyolefin composition is generally selected independently of the first polyolefin composition. The second polyolefin composition comprises the second polyethylene resin and the second polypropylene resin.

In an embodiment, the method for preparing the second polyolefin solution differs from the method for preparing the first polyolefin solution, only in that the mixing temperature is preferably in a range from the melting point (Tm2) of the second polypropylene to Tm2+90° C.

In an embodiment, the first polyethylene resin is present in the first polyolefin solution in an amount in the range of from about 0.5 wt. % to about 75 wt. % based on the total weight of polyolefin in the first polyolefin solution, and the first polyolefin solution optionally comprises a first polypropylene resin, the polypropylene resin being present in the first polyolefin solution in an amount in the range of from about 0 wt. % to about 10 wt. % based on the total weight of polyolefin in the first polyolefin solution.

In an embodiment, the second polypropylene resin is present in the second polyolefin solution in an amount in the range of from about 10 wt. % to about 60 wt. % based on the total weight of polyolefin in the second polyolefin solution, and the second polyolefin solution optionally comprises a second polyethylene resin, the second polyethylene resin being present in the second polyolefin solution in an amount in the range of from about 40 wt. % to about 90 wt. % based on the total weight of polyolefin in the second polyolefin solution.

(3) Extrusion

In an embodiment, the first polyolefin solution is conducted from a first extruder to a first die and the second polyolefin solution is conducted from a second extruder to a second die. A layered extrudate in sheet form (i.e., a body significantly larger in the planar directions than in the thickness direction) can be extruded from the first and second die. Optionally, the first and second polyolefin solutions are co-extruded from the first and second die with a planar surface of a first extrudate layer formed from the first polyolefin solution in contact with a planar surface of a second extrudate layer formed from the second polyolefin solution. A planar surface of the extrudate can be defined by a first vector in the machine direction of the extrudate and a second vector in the transverse direction of the extrudate.

In an embodiment, a die assembly is used where the die assembly comprises the first and second die, as for example when the first die and the second die share a common partition between a region in the die assembly containing the first polyolefin solution and a second region in the die assembly containing the second polyolefin solution.

In another embodiment, a plurality of dies is used, with each die connected to an extruder for conducting either the first or second polyolefin solution to the die. For example, in one embodiment, the first extruder containing the first polyolefin solution is connected to a first die and a third die, and a second extruder containing the second polyolefin solution is connected to a second die. As is the case in the preceding embodiment, the resulting layered extrudate can be co-extruded from the first, second, and third die (e.g., simultaneously) to form a three-layer extrudate comprising a first and a third layer constituting surface layers (e.g., top and bottom layers) produced from the first polyolefin solution; and a second layer constituting a middle or intermediate layer of the extrudate situated between and in planar contact with both surface layers, where the second layer is produced from the second polyolefin solution.

In yet another embodiment, the same die assembly is used but with the polyolefin solutions reversed, i.e., the second extruder containing the second polyolefin solution is connected to the first die and the third die, and the first extruder containing the first polyolefin solution is connected to the second die.

In any of the preceding embodiments, die extrusion can be conducted using conventional die extrusion equipment. For example, extrusion can be conducted by a flat die or an inflation die. In one embodiment useful for co-extrusion of multi-layer gel-like sheets, multi-manifold extrusion can be used, in which the first and second polyolefin solutions are conducted to separate manifolds in a multi-layer extrusion die and laminated at a die lip inlet. In another such embodiment, block extrusion can be used, in which the first and second polyolefin solutions are first combined into a laminar flow (i.e., in advance), with the laminar flow then connected to a die. Because multi-manifold and block processes are known to those skilled in the art of processing polyolefin films (e.g., as disclosed in JP06-122142 A, JP06-106599A), they are deemed conventional, therefore, their operation will be not described in detail.

Die selection is not critical, and, e.g., a conventional multi-layer-sheet-forming, flat or inflation die can be used. Die gap is not critical. For example, the multi-layer-sheet-forming flat die can have a die gap of about 0.1 mm to about 5 mm. Die temperature and extruding speed are also non-critical parameters. For example, the die can be heated to a die temperature ranging from about 140° C. to about 250° C. during extrusion. The extruding speed can range, for example, from about 0.2 m/minute to about 15 m/minute. The thickness of the layers of the layered extrudate can be independently selected. For example, the gel-like sheet can have relatively thick surface layers (or “skin” layers) compared to the thickness of an intermediate layer of the layered extrudate.

While the extrusion has been described in terms of embodiments producing two and three-layer extrudates, the extrusion step is not limited thereto. For example, a plurality of dies and/or die assemblies can be used to produce multi-layer extrudates having four or more layers using the extrusion methods of the preceding embodiments. In such a layered extrudate, each surface or intermediate layer can be produced using either the first polyolefin solution and/or the second polyolefin solution.

(4) Formation of a Multi-Layer, Gel-Like Sheet

The multi-layer extrudate can be formed into a multi-layer, gel-like sheet by cooling, for example. Cooling rate and cooling temperature are not particularly critical. For example, the multi-layer, gel-like sheet can be cooled at a cooling rate of at least about 50° C./minute until the temperature of the multi-layer, gel-like sheet (the cooling temperature) is approximately equal to the multi-layer, gel-like sheet's gelation temperature (or lower). In an embodiment, the extrudate is cooled by exposing the extrudate to a temperature of about 25° C. or lower in order to form the multi-layer gel-like sheet. While not wishing to be bound by any theory or model, it is believed that cooling the layered extrudate sets the polyolefin micro-phases of the first and second polyolefin solutions for separation by the membrane-forming solvent or solvents. It has been observed that in general a slower cooling rate (e.g., less than 50° C./minute) provides the multi-layer, gel-like sheet with larger pseudo-cell units, resulting in a coarser higher-order structure. On the other hand, a relatively faster cooling rate (e.g., 80° C./minute) results in denser cell units. Although it is not a critical parameter, when the cooling rate of the extrudate is less than 50° C./minute, increased polyolefin crystallinity in the layer can result, which can make it more difficult to process the multi-layer, gel-like sheet in subsequent stretching steps. The choice of cooling method is not critical. For example conventional sheet cooling methods can be used. In an embodiment, the cooling method comprises contacting the layered extrudate with a cooling medium such as cooling air, cooling water, etc. Alternatively, the extrudate can be cooled via contact with rollers cooled by a cooling medium, etc.

(5) Removal of the First and Second Membrane-Forming Solvents

In an embodiment, at least a portion of the first and second membrane-forming solvents are removed (or displaced) from the multi-layer gel-like sheet in order to form a solvent-removed gel-like sheet. A displacing (or “washing”) solvent can be used to remove (wash away, or displace) the first and second membrane-forming solvents. While not wishing to be bound by any theory or model, it is believed that because the polyolefin phases in the multi-layer gel-like sheet produced from the first polyolefin solution and the second polyolefin solution (i.e., the first polyolefin and the second polyolefin) are separated from the membrane-forming solvent phase, the removal of the membrane-forming solvent provides a porous membrane constituted by fibrils forming a fine three-dimensional network structure and having pores communicating three-dimensionally and irregularly. The choice of washing solvent is not critical provided it is capable of dissolving or displacing at least a portion of the first and/or second membrane-forming solvent. Suitable washing solvents include, for instance, one or more of volatile solvents such as saturated hydrocarbons such as pentane, hexane, heptane, etc.; chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, etc.; ethers such as diethyl ether, dioxane, etc.; ketones such as methyl ethyl ketone, etc.; linear fluorocarbons such as trifluoroethane, C₆F₁₄, C₇F₁₆, etc.; cyclic hydrofluorocarbons such as C₅H₃F₇, etc.; hydrofluoroethers such as C₄F₉OCH₃, C₄F₉OC₂H₅, etc.; and perfluoroethers such as C₄F₉OCF₃, C₄F₉OC₂F₅, etc.

The method for removing the membrane-forming solvent is not critical, and any method capable of removing a significant amount of solvent can be used, including conventional solvent-removal methods. For example, the multi-layer, gel-like sheet can be washed by immersing the sheet in the washing solvent and/or showering the sheet with the washing solvent. The amount of washing solvent used is not critical, and will generally depend on the method selected for removal of the membrane-forming solvent. For example, the amount of washing solvent used can range from about 300 to about 30,000 parts by mass, based on the mass of the gel-like sheet. While the amount of membrane-forming solvent removed is not particularly critical, generally a higher quality (more porous) membrane will result when at least a major amount of first and second membrane-forming solvent is removed from the gel-like sheet. In an embodiment, the membrane-forming solvent is removed from the gel-like sheet (e.g., by washing) until the amount of the remaining membrane-forming solvent in the multi-layer gel-like sheet becomes less than 1 wt. %, based on the weight of the gel-like sheet.

(6) Drying of the Solvent-Removed Gel-Like Sheet

In an embodiment, the solvent-removed multi-layer, gel-like sheet obtained by removing at least a portion of the membrane-forming solvent is dried in order to remove the washing solvent. Any method capable of removing the washing solvent can be used, including conventional methods such as heat-drying, wind-drying (moving air), etc. The temperature to which the gel-like sheet is exposed during drying (i.e., drying temperature) is not critical. For example, the drying temperature can be equal to or lower than the crystal dispersion temperature Tcd. Tcd is the lower of the crystal dispersion temperature Tcd₁ of the first polyethylene resin and the crystal dispersion temperature Tcd₂ of the second polyethylene resin (when used). For example, the drying temperature can be at least 5° C. below the crystal dispersion temperature Tcd. The crystal dispersion temperature of the first and second polyethylene resin can be determined by measuring the temperature characteristics of the kinetic viscoelasticity of the polyethylene resin according to ASTM D 4065. In an embodiment, at least one of the first or second polyethylene resins have a crystal dispersion temperature in the range of about 90° C. to about 100° C.

Although it is not critical, drying can be conducted until the amount of remaining washing solvent is about 5 wt. % or less on a dry basis, i.e., based on the weight of the dry multi-layer, microporous polyolefin membrane. In another embodiment, drying is conducted until the amount of remaining washing solvent is about 3 wt. % or less on a dry basis. Insufficient drying can be recognized because it generally leads to an undesirable decrease in the porosity of the multi-layer, microporous membrane. If this is observed, an increased drying temperature and/or drying time should be used. Removal of the washing solvent, e.g., by drying or otherwise, results in the formation of the multi-layer, microporous polyolefin membrane.

(7) Stretching

Prior to the step for removing the membrane-forming solvents (namely prior to step 5), the multi-layer, gel-like sheet can be stretched in order to obtain a stretched, multi-layer, gel-like sheet. It is believed that the presence of the first and second membrane-forming solvents in the multi-layer, gel-like sheet results in a relatively uniform stretching magnification. Heating the multi-layer, gel-like sheet, especially at the start of stretching or in a relatively early stage of stretching (e.g., before 50% of the stretching has been completed) is also believed to aid the uniformity of stretching.

Neither the choice of stretching method nor the degree of stretching magnification is particularly critical. For example, any method capable of stretching the multi-layer, gel-like sheet to a predetermined magnification (including any optional heating) can be used. In an embodiment, the stretching can be accomplished by one or more of tenter-stretching, roller-stretching, or inflation stretching (e.g., with air). Although the choice is not critical, the stretching can be conducted monoaxially (i.e., in either the machine or transverse direction) or biaxially (both the machine or transverse direction). In an embodiment, biaxial stretching is used. In the case of biaxial stretching (also called biaxial orientation), the stretching can be simultaneous biaxial stretching, sequential stretching along one planar axis and then the other (e.g., first in the transverse direction and then in the machine direction), or multi-stage stretching (for instance, a combination of the simultaneous biaxial stretching and the sequential stretching). In an embodiment, simultaneous biaxial stretching is used.

The stretching magnification is not critical. In an embodiment where monoaxial stretching is used, the linear stretching magnification can be, e.g., about 2 fold or more, or about 3 to about 30 fold. In an embodiment where biaxial stretching is used, the linear stretching magnification can be, e.g., about 3 fold or more in any planar direction. In another embodiment, the area magnification resulting from stretching is at least about 9 fold, or at least about 16 fold, or at least about 25 fold. Although it is not a critical parameter, when the stretching results in an area magnification of at least about 9 fold, the multi-layer microporous polyolefin membrane has a relatively higher pin puncture strength. When attempting an area magnification of more than about 400 fold, it can be more difficult to operate the stretching apparatus.

The temperature to which the multi-layer, gel-like sheet is exposed during stretching (namely the stretching temperature) is not critical. In an embodiment, the temperature of the gel-like sheet during stretching can be about (Tm+10° C.) or lower, or optionally in a range that is higher than Tcd but lower than Tm, wherein Tm is the lesser of the melting point Tm₁ of the first polyethylene and the melting point Tm₂ of the second polyethylene (when used). Although this parameter is not critical, when the stretching temperature is higher than approximately the melting point Tm+10° C., at least one of the first or second polyethylene can be in the molten state, which can make it more difficult to orient the molecular chains of the polyolefin in the multi-layer gel-like sheet during stretching. And when the stretching temperature is lower than approximately Tcd, at least one of the first or second polyethylene can be so insufficiently softened that it is difficult to stretch the multi-layer, gel-like sheet without breakage or tears, which can result in a failure to achieve the desired stretching magnification. In an embodiment, the stretching temperature ranges from about 90° C. to about 140° C., or from about 100° C. to about 130° C.

While not wishing to be bound by any theory or model, it is believed that such stretching causes cleavage between polyethylene lamellas, making the polyethylene phases finer and forming large numbers of fibrils. The fibrils form a three-dimensional network structure (three-dimensionally irregularly connected network structure). Consequently, the stretching when used generally makes it easier to produce a relatively high-mechanical strength multi-layer, microporous polyolefin membrane with a relatively large pore size. Such multi-layer, microporous membranes are believed to be particularly suitable for use as battery separators.

Optionally, stretching can be conducted in the presence of a temperature gradient in a thickness direction (i.e., a direction approximately perpendicular to the planar surface of the multi-layer, microporous polyolefin membrane). In this case, it can be easier to produce a multi-layer, microporous polyolefin membrane with improved mechanical strength. The details of this method are described in Japanese Patent 3347854.

(8) Hot Solvent Treatment Step

Although it is not required, the multi-layer, gel-like sheet can be treated with a hot solvent between steps (4) and (5). When used, it is believed that the hot solvent treatment provides the fibrils (such as those formed by stretching the multi-layer gel-like sheet) with a relatively thick leaf-vein-like structure. Such a structure, it is believed, makes it less difficult to produce a multi-layer, microporous membrane having large pores with relatively high strength and permeability. The term “leaf-vein-like” means that the fibrils have thick trunks and thin fibers extending therefrom in a network structure. The details of this method are described in WO 2000/20493.

(9) Stretching of Multi-Layer, Microporous Membrane (“Dry Stretching”)

In an embodiment, the dried multi-layer, microporous membrane of step (6) can be stretched, at least monoaxially. The stretching method selected is not critical, and conventional stretching methods can be used such as by a tenter method, etc. While it is not critical, the membrane can be heated during stretching. While the choice is not critical, the stretching can be monoaxial or biaxial. When biaxial stretching is used, the stretching can be conducted simultaneously in both axial directions, or, alternatively, the multi-layer, microporous polyolefin membrane can be stretched sequentially, e.g., first in the machine direction and then in the transverse direction. In an embodiment, simultaneous biaxial stretching is used. When the multi-layer gel-like sheet has been stretched as described in step (7) the stretching of the dry multi-layer, microporous polyolefin membrane in step (9) can be called dry-stretching, re-stretching, or dry-orientation.

The temperature to which the dry multi-layer, microporous membrane is exposed during stretching (the “dry stretching temperature”) is not critical. In an embodiment, the dry stretching temperature is approximately equal to the melting point Tm or lower, for example in the range of from about the crystal dispersion temperature Tcd to the about the melting point Tm. When the dry stretching temperature is higher than Tm, it can be more difficult to produce a multi-layer, microporous polyolefin membrane having a relatively high compression resistance with relatively uniform air permeability characteristics, particularly in the transverse direction when the dry multi-layer, microporous polyolefin membrane is stretched transversely. When the stretching temperature is lower than Tcd, it can be more difficult to sufficiently soften the first and second polyolefins, which can lead to tearing during stretching, and a lack of uniform stretching. In an embodiment, the dry stretching temperature ranges from about 90° C. to about 135° C., or from about 95° C. to about 130° C.

When dry-stretching is used, the stretching magnification is not critical. For example, the stretching magnification of the multi-layer, microporous membrane can range from about 1.1 fold to about 1.8 fold in at least one planar (e.g., lateral) direction. Thus, in the case of monoaxial stretching, the stretching magnification can range from about 1.1 fold to about 1.8 fold in the longitudinal direction (i.e., the “machine direction”) or the transverse direction, depending on whether the membrane is stretched longitudinally or transversely. Monoaxial stretching can also be accomplished along a planar axis between the longitudinal and transverse directions.

In an embodiment, biaxial stretching is used (i.e., stretching along two planar axis) with a stretching magnification of about 1.1 fold to about 1.8 fold along both stretching axes, e.g., along both the longitudinal and transverse directions. The stretching magnification in the longitudinal direction need not be the same as the stretching magnification in the transverse direction. In other words, in biaxial stretching, the stretching magnifications can be selected independently. In an embodiment, the dry-stretching magnification is the same in both stretching directions. If desired, the membrane can be stretched to a magnification that is larger than 1.8 fold, particularly when during subsequent processing (e.g., heat treatment) the membrane relaxes (or shrinks) in the direction(s) of stretching to a achieve a final magnification of about 1.1 to about 1.8 fold compared to the size of the film at the start of the dry orientation step.

(10) Heat Treatment

In an embodiment, the dried multi-layer, microporous membrane can be heat-treated following step (6). It is believed that heat-treating stabilizes the polyolefin crystals in the dried multi-layer, microporous polyolefin membrane to form uniform lamellas. In an embodiment, the heat treatment comprises heat-setting and/or annealing. When heat-setting is used, it can be conducted using conventional methods such as tenter methods and/or roller methods. Although it is not critical, the temperature to which the dried multi-layer, microporous polyolefin membrane is exposed during heat-setting (i.e., the “heat-setting temperature”) can range from the Tcd to about the Tm. In an embodiment, the heat-setting temperature ranges from about the dry stretching temperature of the multi-layer, microporous polyolefin membrane ±5° C., or about the dry stretching temperature of the multi-layer, microporous polyolefin membrane ±3° C.

Annealing differs from heat-setting in that it is a heat treatment with no load applied to the multi-layer, microporous polyolefin membrane. The choice of annealing method is not critical, and it can be conducted, for example, by using a heating chamber with a belt conveyer or an air-floating-type heating chamber. Alternatively, the annealing can be conducted after the heat-setting with the tenter clips slackened. The temperature of the multi-layer, microporous polyolefin membrane during annealing (i.e., the annealing temperature) is not critical. In an embodiment, the annealing temperature ranges from about the melting point Tm or lower, or in a range from about 60° C. to (Tm-10° C.). It is believed that annealing makes it less difficult to produce a multi-layer, microporous polyolefin membrane having relatively high permeability and strength.

(11) Cross-Linking

In an embodiment, the multi-layer, microporous polyolefin membrane can be cross-linked (e.g., by ionizing radiation rays such as α-rays, β-rays, γ-rays, electron beams, etc.) after step (6). For example, when irradiating electron beams are used for cross-linking, the amount of electron beam radiation can be about 0.1 Mrad to about 100 Mrad, using an accelerating voltage in the range of about 100 kV to about 300 kV. It is believed that the cross-linking treatment makes it less difficult to produce a multi-layer, microporous polyolefin membrane with relatively high meltdown temperature.

(12) Hydrophilizing Treatment

In an embodiment, the multi-layer, microporous polyolefin membrane can be subjected to a hydrophilic treatment (i.e., a treatment which makes the multi-layer, microporous polyolefin membrane more hydrophilic). The hydrophilic treatment can be, for example, a monomer-grafting treatment, a surfactant treatment, a corona-discharging treatment, etc. In an embodiment, the monomer-grafting treatment is used after the cross-linking treatment.

When a surfactant treatment is used, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, for example, either alone or in combination. In an embodiment, a nonionic surfactant is used. The choice of surfactant is not critical. For example, the multi-layer, microporous polyolefin membrane can be dipped in a solution of the surfactant and water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the solution, e.g., by a doctor blade method.

B. Second Production Method

The second method for producing the multi-layer, microporous polyolefin membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a first membrane-forming solvent to prepare a first polyolefin solution, (2) combining a second polyolefin composition and a second membrane-forming solvent to prepare a second polyolefin solution, (3) extruding the first polyolefin solution through a first die and the second solution through a second die and then laminating the extruded first and second polyolefin solutions to form a multi-layer extrudate, (4) cooling the multi-layer extrudate to form a multi-layer, gel-like sheet, (5) removing at least a portion of the membrane-forming solvent from the multi-layer, gel-like sheet to form a solvent-removed gel-like sheet, and (6) drying the solvent-removed gel-like sheet in order to form the multi-layer, microporous membrane. An optional stretching step (7), and an optional hot solvent treatment step (8), etc., can be conducted between steps (4) and (5), if desired. After step (6), an optional step (9) of stretching a multi-layer, microporous membrane, an optional heat treatment step (10), an optional cross-linking step with ionizing radiations (11), and an optional hydrophilic treatment step (12), etc., can be conducted.

The process steps and conditions of the second production method are generally the same as those of the analogous steps described in connection with the first production method, except for step (3). Consequently, step (3) will be explained in more detail.

The type of die used is not critical provided the die is capable of forming an extrudate that can be laminated. In one embodiment, sheet dies (which can be adjacent or connected) are used to form the extrudates. The first and second sheet dies are connected to first and second extruders, respectively, where the first extruder contains the first polyolefin solution and the second extruder contains the second polyolefin solution. While not critical, lamination is generally easier to accomplish when the extruded first and second polyolefin solution are still at approximately the extrusion temperature. The other conditions may be the same as in the first method.

In another embodiment, the first, second, and third sheet dies are connected to first, second and third extruders, where the first and third sheet dies contain the first polyolefin solutions, and the second sheet die contains the second polyolefin solution. In this embodiment, a laminated extrudate is formed constituting outer layers comprising the extruded first polyolefin solution and one intermediate comprising the extruded second polyolefin solution.

In yet another embodiment, the first, second, and third sheet dies are connected to first, second, and third extruders, where the second sheet die contains the first polyolefin solution, and the first and third sheet dies contain the second polyolefin solution. In this embodiment, a laminated extrudate is formed constituting outer layers comprising the extruded second polyolefin solution and one intermediate comprising extruded first polyolefin solution.

C. Third Production Method

The third method for producing the multi-layer, microporous polyolefin membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a membrane-forming solvent to prepare a first polyolefin solution, (2) combining a second polyolefin composition and a second membrane-forming solvent to prepare a second polyolefin solution, (3) extruding the first polyolefin solution through at least one first die to form at least one first extrudate, (4) extruding the second polyolefin solution through at least one second die to form at least one second extrudate, (5) cooling first and second extrudates to form at least one first gel-like sheet and at least one second gel-like sheet, (6) laminating the first and second gel-like sheet to form a multi-layer, gel-like sheet, (7) removing at least a portion of the membrane-forming solvent from the resultant multi-layer, gel-like sheet to form a solvent-removed gel-like sheet, and (8) drying the solvent-removed gel-like sheet in order to form the multi-layer, microporous membrane. An optional stretching step (9), and an optional hot solvent treatment step (10), etc., can be conducted between steps (5) and (6) or between steps (6) and (7), if desired. After step (8), an optional step (11) of stretching a multi-layer, microporous membrane, an optional heat treatment step (12), an optional cross-linking step with ionizing radiations (13), and an optional hydrophilic treatment step (14), etc., can be conducted.

The main difference between the third production method and the second production method is in the order of the steps for laminating and cooling.

In the second production method, laminating the first and second polyolefin solutions is conducted before the cooling step. In the third production method, the first and second polyolefin solutions are cooled before the laminating step.

The steps of (1), (2), (7) and (8) in the third production method can be the same as the steps of (1), (2), (5) and (6) in the first production method as described above. For the extrusion of the first polyolefin solution through the first die, the conditions of step (3) of the second production method can be used for step (3) of the third production method. For the extrusion of the second solution through the second die, the conditions of step (4) in the third production method can be the same as the conditions of step (3) in the second production method. In one embodiment, either the first or second polyolefin solution is extruded through a third die. In this way, a multi-layer laminate can be formed having two layers produced from the first polyolefin solution and a single layer produced from the second polyolefin solution, or vice versa.

Step (5) of the third production method can be the same as step (4) in the first production method except that in the third production method the first and second gel-like sheets are formed separately.

The step (6) of laminating the first and second gel-like sheets will now be explained in more detail. The choice of lamination method is not particularly critical, and conventional lamination methods such as heat-induced lamination can be used to laminate the multi-layer gel-like sheet. Other suitable lamination methods include, for example, heat-sealing, impulse-sealing, ultrasonic-bonding, etc., either alone or in combination. Heat-sealing can be conducted using, e.g., one or more pair of heated rollers where the gel-like sheets are conducted through at least one pair of the heated rollers. Although the heat-sealing temperature and pressure are not particularly critical, sufficient heating and pressure should be applied for a sufficient time to ensure that the gel-like sheets are appropriately bonded to provide a multi-layer, microporous membrane with relatively uniform properties and little tendency toward delamination. In an embodiment, the heat-sealing temperature can be, for instance, about 90° C. to about 135° C., or from about 90° C. to about 115° C. In an embodiment, the heat-sealing pressure can be from about 0.01 MPa to about −50 MPa.

As is the case in the first and second production method, the thickness of the layers formed from the first and second polyolefin solution (i.e., the layers comprising the first and second microporous layer materials) can be controlled by adjusting the thickness of the first and second gel-like sheets and by the amount of stretching (stretching magnification and dry stretching magnification), when one or more stretching steps are used. Optionally, the lamination step can be combined with a stretching step by passing the gel-like sheets through multi-stages of heated rollers.

In an embodiment, the third production method forms a multi-layer, polyolefin gel-like sheet having at least three layers. For example, after cooling two extruded first polyolefin solutions and one extruded second polyolefin solution to form the gel-like sheets, the multi-layer gel-like sheet can be laminated with outer layers comprising the extruded first polyolefin solution and an intermediate layer comprising the extruded second polyolefin solution. In another embodiment, after cooling two extruded second polyolefin solutions and one extruded first polyolefin solution to form the gel-like sheets, the multi-layer gel-like sheet can be laminated with outer layers comprising the extruded second polyolefin solution and an intermediate layer comprising the extruded first polyolefin solution.

The stretching step (9) and the hot solvent treatment step (10) can be the same as the stretching step (7) and the hot solvent treatment step (8) as described for the first production method, except stretching step (9) and hot solvent treatment step (10) are conducted on the first and/or second gel-like sheets. The stretching temperatures of the first and second gel-like sheets are not critical. For example, the stretching temperatures of the first gel-like sheet can be, e.g., Tm₁+10° C. or lower, or optionally about Tcd₁ or higher but lower than about Tm₁. The stretching temperature of the second gel-like sheet can be, e.g., Tm₂+10° C. or lower, or optionally about Tcd₂ or higher but lower than about Tm₂.

D. Fourth Production Method

The fourth method for producing the multi-layer, microporous polyolefin membrane comprises the steps of (1) combining (e.g., by melt-blending) a first polyolefin composition and a membrane-forming solvent to prepare a first polyolefin solution, (2) combining a second polyolefin composition and a second membrane-forming solvent to prepare a second polyolefin solution, (3) extruding the first polyolefin solution through at least one first die to form at least one first extrudate, (4) extruding the second polyolefin solution through at least one second die to form at least one second extrudate, (5) cooling first and second extrudates to form at least one first gel-like sheet and at least one second gel-like sheet, (6) removing at least a portion of the first and second membrane-forming solvents from the first and second gel-like sheets to form solvent-removed first and second gel-like sheets, (7) drying the solvent-removed first and second gel-like sheets to form at least one first polyolefin membrane and at least one second polyolefin membrane, and (8) laminating the first and second microporous polyolefin membranes in order to form the multi-layer, microporous polyolefin membrane.

A stretching step (9), a hot solvent treatment step (10), etc., can be conducted between steps (5) and (6), if desired. A stretching step (11), a heat treatment step (12), etc., can be conducted between steps (7) and (8), if desired. After step (8), a step (13) of stretching a multi-layer, microporous membrane, a heat treatment step (14), a cross-linking step with ionizing radiations (15), a hydrophilic treatment step (16), etc., can be conducted if desired.

Steps (1) and (2) in the fourth production method can be conducted under the same conditions as steps of (1) and (2) in the first production method. Steps (3), (4), and (5) in the fourth production method can be conducted under the same conditions as steps (3), (4), and (5) in the third method. Step (6) in the fourth production method can be conducted under the same conditions as step (5) in the first production method except for removing the membrane-forming solvent from the first and second gel-like sheets. Step (7) in the fourth production method can be conducted under the same conditions as step (6) in the first production method except that in the fourth production method the first and second solvent-removed gel-like sheets are dried separately. Step (8) in the fourth production method can be conducted under the same conditions as the step (6) in the third production method except for laminating the first and second polyolefin microporous membranes. The stretching step (9) and the hot solvent treatment step (10) in the fourth production method can be conducted under the same conditions as step (9) and (10) in the third production method. The stretching step (11) and the heat treatment step (12) in the fourth production method can be conducted under the same conditions as steps (9) and (10) in the first production method except that in the fourth production method the first and second polyolefin microporous membranes are stretched and/or heat treated.

In an embodiment, in the stretching step (11) in the fourth production method, the stretching temperature of the first polyolefin microporous membranes can be about Tm₁ or lower, or optionally about Tcd₁ to about Tm₁, and the stretching temperature of the second polyolefin microporous membrane can be about Tm₂ or lower, or optionally about Tcd₂ to about Tm₂.

In an embodiment, the heat treatment step (12) in the fourth production method can be HS and/or annealing. For example, in the heat treatment step (12) in the fourth production method, the heat-setting temperature of the first polyolefin microporous membranes can be about Tcd₁ to about Tm₁, or optionally about the dry stretching temperature ±5° C., or optionally about the dry stretching temperature ±3° C. In an embodiment, in the heat treatment step (12) in the fourth production method, the heat-setting temperature of the second microporous membrane can be about Tcd₂ to about Tm₂, or optionally the dry stretching temperature ±5° C., or optionally the dry stretching temperature ±3° C. When the HS is used, it can be conducted by, e.g., a tenter method or a roller method.

In an embodiment, in the heat treatment step (12) in the fourth production method, the annealing temperature of the first microporous membrane can be about Tm₁ or lower, or optionally about 60° C. to about (Tm₁−10° C.). In an embodiment, in the heat treatment step (12) in the fourth production method, the annealing temperature of the second microporous membranes can be about Tm₂ or lower, or optionally about 60° C. to about (Tm₂−10° C.).

The conditions in step (13), stretching a multi-layer, microporous membrane, a heat treatment step (14), a cross-linking step with ionizing radiations (15), and a hydrophilic treatment step (16) in the fourth production method can be the same as those for steps (9), (10), (11) and (12) in the first production method.

[4] The Properties of a Multi-Layer, Microporous Polyolefin Membrane

In an embodiment, the multi-layer, microporous polyolefin membrane has a thickness ranging from about 3 μm to about 200 μm, or about 5 μm to about 50 μm. Optionally, when the membrane is a multi-layer, microporous polyolefin membrane, it has one or more of the following characteristics.

A. Porosity of about 25% to about 80%

When the porosity is less than 25%, the multi-layer, microporous polyolefin membrane generally does not exhibit the desired air permeability for use as a battery separator. When the porosity exceeds 80%, it is more difficult to produce a battery separator of the desired strength, which can increase the likelihood of internal electrode short-circuiting. In an embodiment, the membrane is characterized by or has a least one layer characterized by a hybrid structure, i.e., a relatively broad distribution of differential pore volume as a function of pore diameter. For example, the differential pore volume as a function of pore diameter can have two or more peaks or modes.

B. Weight Per Unit Area of about 5 g/m² to 19 g/m² at 25-μm Thickness

When the basis weight of the multi-layer, microporous polyolefin membrane ranges from about 5 g/m² to 19 g/m² at 25-μm thickness, the membrane has appropriate porosity.

C. Air Permeability of about 20 seconds/100 cm³ to about 700 Seconds/100 cm³ (Normalized to Value at 25-μm Thickness)

In an embodiment, the membrane's normalized air permeability is ≦350 seconds/100 cm³, e.g. in the range of 100 seconds/100 cm³ to 340 seconds/100 cm³. When the normalized air permeability of the membrane (as measured according to JIS P8117) ranges from about 20 seconds/100 cm³ to about 700 seconds/100 cm³, it is less difficult to form batteries having the desired charge storage capacity and desired cyclability. When the air permeability is less than about 20 seconds/100 cm³, it is more difficult to produce a battery having the desired shutdown characteristics, particularly when the temperatures inside the batteries are elevated. Air permeability P₁ measured on a multi-layer, microporous membrane having a thickness T₁ (in μm) according to JIS P8117 can be normalized to air permeability AP₂ at a thickness of 25 μm by the equation of AP₂=(AP₁×25)/T₁.

In an embodiment, the membrane has Normalized Air Permeability satisfying the relationship A≦(M*P)−I where A is the microporous membrane's Normalized Air Permeability expressed in units of sec/100 cm³ and normalized to a 25 μm membrane thickness and P is the microporous membrane's Normalized Pin Puncture Strength expressed in units of mN and normalized to a 25 μm membrane thickness. M is a slope (using the axes and units of FIG. 1) in the range of about 0.09 to about 0.1, or about 0.95 to about 0.99. In an embodiment, M is equal to 0.097. “I” is an intercept on the Y axis (using the axes and units of FIG. 1) that is ≧100, e.g., ≧110, such as ≧ or 150, or ≧200, or ≧250; or, e.g., in the range of about 100 to about 250, such as from about 110 to about 240.

In another embodiment, the membrane has a Normalized Air Permeability and Normalized Pin Puncture Strength that fall on or within the boundary of the ellipse shown in FIG. 1. In yet another embodiment, the membrane has an Air Permeability satisfying the relationship (M₂*P)−I₂≦A≦(M₁*P)−I₁.

M₁ and M₂ are independently selected and can each be, e.g., in the range of about 0.09 to about 0.1, or about 0.95 to about 0.99. In an embodiment, M₁ and M₂ are equal. For example, M₁ and M₂ can be 0.097; and “I₁” can be, e.g., in the range of about 100 to about 240, such as from about 110 to about 230. In an embodiment, I₁ is 110. “I₂” can be, e.g., ≧260. For example, I₂ can be in the range of about 260 to about 450. Units and axes are the same as in FIG. 1.

D. Pin Puncture Strength of about 3,000 mN/25 μm or More

The pin puncture strength (normalized to the value at a 25-μm membrane thickness) is the maximum load measured when the multi-layer, microporous polyolefin membrane is pricked with a needle 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. When the pin puncture strength of the multi-layer, microporous polyolefin membrane is less than 3,000 mN/25 μm, it is more difficult to produce a battery having the desired mechanical integrity, durability, and toughness. In an embodiment, the membrane's Normalized Pin Puncture strength is in the range of 4500 mN/25μ to 600 mN/25 g.

E. Heat Shrinkage Ratio of 10% or Less

When the heat shrinkage ratio measured after holding the multi-layer, microporous membrane at a temperature of about 105° C. for 8 hours exceeds 10% in both longitudinal and transverse directions, it is more difficult to produce a battery that will not exhibit internal short-circuiting when the heat generated in the battery results in the shrinkage of the separators. In an embodiment, the membrane's heat shrinkage at 105° C. is in the range of 1% to 5% (MD) and 1% to 5% (TD), such as 1% to 3% (TD).

F. Shutdown Temperature of about 140° C. or Lower

When the shutdown temperature of the multi-layer, microporous polyolefin membrane exceeds 140° C., it is more difficult to produce a battery separator with the desired shutdown response when the battery is overheated. One way to determine shutdown temperature involves determining the temperature at a point of inflection observed near the melting point of the multi-layer, microporous polyolefin membrane, under the condition that a test piece of 3 mm in the longitudinal direction and 10 mm in the transverse direction is heated from room temperature at a speed of 5° C./minute while drawing the test piece in the longitudinal direction under a load of 2 g. In an embodiment, the shutdown temperature is in the range of about 120-140° C.

G. Rupture Temperature of at least about 180° C.

The microporous membrane should have a rupture temperature of about 180° C. or higher, or about 185° C. or higher, or about 190° C. or higher. In an embodiment, the rupture temperature is in the range of about 180° C. to about 195° C., or about 185° C. to about 190° C. Rupture temperature can be measured as follows. A microporous membrane of 5 cm×5 cm is sandwiched by blocks each having a circular opening of 12 mm in diameter, and a tungsten carbide ball of 10 mm in diameter was placed on the microporous membrane in the circular opening. While heating at a temperature-elevating speed of 5° C./minute, the temperature at which the microporous polyolefin membrane is ruptured by melting is measured and recorded as the Rupture temperature. FIG. 2 shows that when the total amount polypropylene in the membrane is 2 wt. % or higher, based on the total weight of the membrane, the membrane's Rupture temperature 180° C. or higher. In an embodiment, the total amount polypropylene in the membrane having

-   (a) an Mw of 600,000 or higher, -   (b) a molecular weight distribution in the range of 3 to 10, and -   (c) a heat of fusion of 90 J/g or higher -   is 2 wt. % or higher, or 2.05 wt. % or higher, or 2.1 wt. % or     higher, based on the total weight of the membrane.

H. Maximum Shrinkage in Molten State of 30% or Less

The multi-layer microporous membrane should exhibit a maximum shrinkage in the molten state (about 140° C.) of about 30% or less, preferably about 20% or less as measured by a thermomechanical analyzer, (“TMA”).

[5] Battery Separator

In and embodiment, the battery separator formed by the above multi-layer, microporous polyolefin membrane has a thickness in the range of about 3 μm to about 200 μm, or about 5 μm to about 50 μm. Depending, e.g., on the choice of electrolyte, separator swelling might increase the final thickness to a value larger than 200 μm.

[6] Battery

In an embodiment, the multi-layer, microporous polyolefin membrane can be used as a separator for primary and secondary batteries such as lithium ion batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, and particularly for lithium ion secondary batteries. Explanations will be made below on the lithium ion secondary batteries.

The lithium secondary battery comprises a cathode, an anode, and a separator located between the anode and the cathode. The separator generally contains an electrolytic solution (electrolyte). The electrode structure is not critical, and conventional electrode structures can be used. The electrode structure may be, for instance, a coin type in which a disc-shaped cathode and anode are opposing, a laminate type in which a planar cathode and anode are alternately laminated with at least one separator situated between the anode and the cathode, a toroidal type in which ribbon-shaped cathode and anode are wound, etc.

The cathode generally comprises a current collector, and a cathodic-active material layer capable of absorbing and discharging lithium ions, which is formed on the current collector. The cathodic-active materials can be, e.g., inorganic compounds such as transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), transition metal sulfides, etc. The transition metals can be, e.g., V, Mn, Fe, Co, Ni, etc. In an embodiment, the lithium composite oxides are lithium nickelate, lithium cobaltate, lithium manganate, laminar lithium composite oxides based on α-NaFeO₂, etc. The anode generally comprises a current collector, and a negative-electrode active material layer formed on the current collector. The negative-electrode active materials can be, e.g., carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, etc.

The electrolytic solutions can be obtained by dissolving lithium salts in organic solvents. The choice of solvent and/or lithium salt is not critical, and conventional solvents and salts can be used. The lithium salts can be, e.g., LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, lower aliphatic carboxylates of lithium, LiAlCl₄, etc. The lithium salts may be used alone or in combination. The organic solvents can be organic solvents having relatively high boiling points (compared to the battery's shut-down temperature) and high dielectric constants. Suitable organic solvents include ethylene carbonate, propylene carbonate, ethylmethyl carbonate, γ-butyrolactone, etc.; organic solvents having low boiling points and low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, diethyl carbonate, and the like, including mixtures thereof. Because the organic solvents generally having high dielectric constants generally also have a high viscosity, and vice versa, mixtures of high- and low-viscosity solvents can be used.

When the battery is assembled, the separator is generally impregnated with the electrolytic solution, so that the separator (multi-layer, microporous membrane) is provided with ion permeability. The choice of impregnation method is not critical, and conventional impregnation methods can be used. For example, the impregnation treatment can be conducted by immersing the multi-layer, microporous membrane in an electrolytic solution at room temperature.

The method selected for assembling the battery is not critical, and conventional battery-assembly methods can be used. For example, when a cylindrical battery is assembled, a cathode sheet, a separator formed by the multi-layer, microporous membrane and an anode sheet are laminated in this order, and the resultant laminate is wound to a toroidal-type electrode assembly. A second separator might be needed to prevent short-circuiting of the toroidal windings. The resultant electrode assembly can be deposited into a battery can and then impregnated with the above electrolytic solution, and a battery lid acting as a cathode terminal provided with a safety valve can be caulked to the battery can via a gasket to produce a battery.

[7] Examples

The present invention will be explained in more detail referring to the following non-limiting examples.

Example 1 (1) Preparation of First Polyolefin Solution

A first polyolefin composition comprising (a) 80% of PE1 having a weight average molecular weight of 5.6×10⁵ and a molecular weight distribution of 4.05, (b) 20% of PE2 having a weight average molecular weight of 1.9×10⁶ and a molecular weight distribution of 5.09, is prepared by dry-blending. The polyethylene composition has a melting point of 135° C. and a crystal dispersion temperature of 100° C.

Twenty-five parts by weight of the resultant first polyolefin composition is charged into a strong-blending double-screw extruder having an inner diameter of 58 mm and L/D of 42, and 65 parts by mass of liquid paraffin (50 cst at 40° C.) is supplied to the double-screw extruder via a side feeder. Melt-blending is conducted at 210° C. and 200 rpm to prepare a first polyolefin solution.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution is prepared in the same manner as above except as follows. A second polyolefin composition comprising (a) 69% of PE1 having a weight average molecular weight of 5.6×10⁵ and a molecular weight distribution of 4.05, and (b) 1% of PE2 having a weight average molecular weight of 1.9×10⁶ and a molecular weight distribution of 5.09, and (c) 30% of polypropylene resin having a weight average molecular weight of 1.6×10⁶, a molecular weight distribution of 5.21 and a heat of fusion of 114.0 J/g, by weight of the second polyolefin composition, is prepared by dry-blending. The polyolefin composition has a melting point of 135° C. and a crystal dispersion temperature of 100° C. Thirty-five parts by weight of the resultant second polyolefin composition is charged into a strong-blending double-screw extruder having an inner diameter of 58 mm and L/D of 42, and 70 parts by mass of liquid paraffin (50 cst at 40° C.) is supplied to the double-screw extruder via a side feeder. Melt-blending is conducted at 210° C. and 200 rpm to prepare a second polyolefin solution.

(3) Production of Membrane

The first and second polyolefin solutions are supplied from their respective double-screw extruders to a three-layer-extruding T-die, and extruded therefrom to form an extrudate (also called a laminate) of first polyolefin solution layer/second polyolefin solution layer/first polyolefin solution layer at a layer thickness ratio of 46/8/46. The extrudate is cooled while passing through cooling rollers controlled at 20° C., to form a three-layer gel-like sheet, which is simultaneously biaxially stretched at 115° C. to a magnification of 5 fold in both machine (longitudinal) and transverse directions by a tenter-stretching machine. The stretched three-layer gel-like sheet is fixed to an aluminum frame of 20 cm×20 cm, immersed in a bath of methylene chloride controlled at 25° C. to remove liquid paraffin with vibration of 100 rpm for 3 minutes, and dried by air flow at room temperature. The dried membrane is re-stretched by a batch-stretching machine to a magnification of 1.4 fold in a transverse direction at 127° C. The re-stretched membrane, which remains fixed to the batch-stretching machine, is heat-set at 127° C. for 10 minutes to produce a three-layer microporous membrane.

Example 2

Example 1 is repeated except the dried membrane is re-stretched to a magnification of 1.6 fold in a transverse direction at 127° C. and contracted to a magnification of 1.4 fold in the direction at 127° C. compared with original size.

Example 3

Example 1 is repeated except the first polyolefin content in the first polyolefin solution is 25%, the second polyolefin composition comprises (a) 64% of PE1, (b) 1% of PE2 and (c) 35% of the second polypropylene, a layer thickness ratio of first microporous membrane/second microporous membrane/first microporous membrane is 47/6/47, the gel-like sheet is stretched at 119° C. and the dried membrane is re-stretched to a magnification of 1.5 fold in a transverse direction at 127° C. and shrank to a magnification of 1.3 fold in the direction at 127° C.

Example 4

Example 3 is repeated except the second polyolefin composition comprises (a) 49% of PE1, (b) 1% of PE2 and (c) 50% of the second polypropylene, a layer thickness ratio of first microporous membrane/second microporous membrane/first microporous membrane is 46/8/46, the gel-like sheet is stretched at 115° C.

Example 5

Example 4 is repeated except the second polyolefin comprises (a) 79% of PE1, (b) 1% of PE2, and (c) 20% of second polypropylene and a layer thickness ratio of first microporous membrane/second microporous membrane/first microporous membrane is 35/30/35.

Example 6

Example 4 is repeated except a layer thickness ratio of first microporous membrane/second microporous membrane/first microporous membrane is 40/20/40.

Example 7

Example 2 is repeated except the first polyolefin content in the first polyolefin solution is 25% and the second polypropylene resin in the second polyolefin composition has a weight average molecular weight of 0.90×10⁶, a molecular weight distribution of 4.5 and a heat of fusion of 106.0 J/g.

Example 8

Example 7 is repeated except the first polyolefin composition comprises (a) 80% of PE1, (b) 12% of PE2 and (c) 8% of the first polypropylene having a weight average molecular weight of 6.6×10⁵, a molecular weight distribution of 1 and a heat of fusion of 103.3 J/g.

Comparative Example 1

Example 1 is repeated except there is no second polyolefin solution. In other words, the membrane is a monolayer membrane produced from the first polyolefin solution. This comparative example also differs from Example 1 in that the dried membrane is not re-stretched.

Comparative Example 2

Example 2 is repeated except there is no first polyolefin solution. In other words, the membrane is a monolayer membrane produced from the second polyolefin solution.

Comparative Example 3

Example 2 is repeated except the first polyolefin content in the first polyolefin solution is 25% and a layer thickness ratio of first microporous membrane/second microporous membrane/first microporous membrane is 47/6/47.

Comparative Example 4

Example 2 is repeated except the second polypropylene resin in the second polyolefin composition has a weight average molecular weight of 0.54×10⁶, a molecular weight distribution of 4.7 and a heat of fusion of 94.0 J/g.

Comparative Example 5

Example 2 is repeated except the second polypropylene resin in the second polyolefin composition has a weight average molecular weight of 1.56×10⁶, a molecular weight distribution of 3.2 and a heat of fusion of 78.40 J/g.

Comparative Example 6

Example 2 is repeated except the second polypropylene resin in the second polyolefin composition has a weight average molecular weight of 2.67×10⁶, a molecular weight distribution of 2.6 and a heat of fusion of 99.4.

Properties

The properties of the multi-layer microporous membranes of Examples 1-8 and Comparative Examples 1-6 are measured by the following methods. The results are shown in Tables 1 and 2.

(1) Average Thickness (μm)

The thickness of each microporous membrane is measured by a contact thickness meter at 10 mm intervals in the area of 10 cm×10 cm of the membrane, and averaged. The thickness meter used is a Litematic made by Mitsutoyo Corporation.

(2) Porosity (%)

Measured by a weight method using the formula: Porosity %=100×(w2−w1)/w2, wherein “w1” is the actual weight of film and “w2” is the assumed weight of 100% polyethylene.

(3) Weight Per Unit Area (g/m²)

A weight per unit area of the membrane is the weight at 1 square meter and calculated based on the weight of above square membrane.

(4) Air Permeability (sec/100 cm³/25 μm)

Air permeability P₁ measured on each microporous membrane having a thickness T₁ according to JIS P8117 is converted to air permeability P₂ at a thickness of 25 μm by the equation of P₂=(P₁×25)/T₁.

(5) Pin Puncture Strength (mN/25 μm)

The maximum load is measured when each microporous membrane having a thickness of T₁ is pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second. The measured maximum load L₁ is converted to the maximum load L₂ at a thickness of 25 μm by the equation of L₂=(L₁×25)/T₁, and is used as pin puncture strength.

(6) Heat Shrinkage Ratio (%)

The shrinkage ratios of each microporous membrane in both longitudinal and transverse directions are measured three times when exposed to 105° C. for 8 hours, and averaged to determine the heat shrinkage ratio.

(7) Shut Down Temperature (° C.)

The shut down temperature is measured as follows: A rectangular sample of 3 mm×50 mm is cut out of the microporous membrane such that the longitudinal direction of the sample is aligned with the transverse direction of the microporous membrane, and set in a thermomechanical analyzer (TMA/SS6000 available from Seiko Instruments, Inc.) at a chuck distance of 10 mm. With a load of 19.6 mN applied to a lower end of the sample, the temperature is elevated at a rate of 5° C./minute to measure its size change. A temperature at a point of inflection observed near the melting point is defined as the shutdown temperature.

(8) Rupture Temperature (° C.)

A microporous membrane of 5 cm×5 cm is sandwiched by blocks each having a circular opening of 12 mm in diameter, and a tungsten carbide ball of 10 mm in diameter was placed on the microporous membrane in the circular opening. While heating at a temperature-elevating speed of 5° C./minute, the temperature at which the microporous polyolefin membrane is ruptured by melting is measured and recorded as the Rupture temperature.

(9) Maximum Shrinkage in Molten State

The maximum shrinkage in the molten state is measured as follows: A rectangular sample of 3 mm×50 mm is cut out of the microporous membrane such that the longitudinal direction of the sample is aligned with the transverse direction of the microporous membrane, and set in a thermomechanical analyzer (TMA/SS6000 available from Seiko Instruments, Inc.) at a chuck distance of 10 mm. With a load of 19.6 mN applied to a lower end of the sample, the temperature is elevated at a rate of 5° C./minute to measure the membrane sample's size change. A size change ratio is calculated relative to the size at 23° C., to obtain a temperature-size change ratio curve. The maximum shrinkage ratio in the molten state is observed in a temperature range of from 135° C. to 145° C.

TABLE 1 No. Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8 Resin composition First Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 80 80 80 80 80 80 80 80 PE2 Mw 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn 8 8 8 8 8 8 8 8 % by mass 20 20 20 20 20 20 20 12 1^(st) PP Mw — — — — — — — 6.6 × 10⁵ Mw/Mn — — — — — — — 11 Heat of fusion(J/g) — — — — — — — 103.3 % by mass — — — — — — — 8 Conc. of PO Comp. % by mass 35 35 25 25 25 25 25 25 Second Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 69 69 64 49 79 49 69 69 PE2 Mw 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn 8 8 8 8 8 8 8 8 % by mass 1 1 1 1 1 1 1 1 2^(nd) PP Mw 1.10 × 10⁶  1.10 × 10⁶  1.10 × 10⁶  1.10 × 10⁶  1.10 × 10⁶  1.10 × 10⁶  0.90 × 10⁶  1.10 × 10⁶  Mw/Mn 5.0 5.0 5.0 5.0 5.0 5.0 4.5 5.0 Heat of fusion(J/g) 114.0 114.0 114.0 114.0 114.0 114.0 106 114.0 % by mass 30 30 35 50 20 50 30 30 Conc. of PO Comp. % by mass 30 30 30 30 30 30 30 30 Production condition Extrudate Layer structure (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) Layer thickness ratio 46/8/46 46/8/46 47/6/47 46/8/46 35/30/35 40/20/40 46/8/46 46/8/46 Total PP content % by mass 2.08 2.08 2.49 5.43 6.79 11.5 3.31 11.9 Stretching of Gel-Like sheet Temperature (° C.) 115 115 119 115 115 115 115 115 Magnification (MD × TD) 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 Stretching of dried membrane Temperature (° C.) 127 127 127 125 125 125 127 127 Magnification (TD) 1.4 1.6 −> 1.4 1.5 −> 1.3 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 Heat setting treatment Temperature(° C.) 127 127 127 125 125 125 127 127 Time (min) 10 10 10 10 10 10 10 10 Properties Average thickness (μm) 25.9 25.6 24.9 25.1 24.7 25.0 25.5 24.7 Normalized Air Permeability 270 262 230 236 211 326 240 289 (sec/100 cm³/25 μm) Porosity % 47.0 46.5 48.7 47.2 49.1 43.3 48.2 47.3 Weight per unit area 13.6 13.6 12.6 13.3 12.3 14.2 13.1 12.9 Normalized Puncture Strength 5586 5468 3826 5292 5133 4732 5194 4508 (mN/25 μm) Heat shrinkage MD/TD (%) 3.5/4.9 3.4/1.3 3.5/1.5 3.3/1.2 3.7/1.3 3.6/1.1 3.3/1.6 3.2/1.2 Shut Down Temp. ° C. 132 132 132 133 132 132 132 133 Rupture Temp. ° C. 187 189 184 191 188 190 185 191 Maximum shrinkage (TMA) % 29 16 12 15 16 14 15 15 Is A ≦ 0.097P-110? Yes yes yes yes yes yes yes yes

TABLE 2 Comp. Ex Comp. Ex Comp. Ex Comp. Ex Comp. Ex Comp. Ex No. 1 2 3 4 5 6 Resin composition First Polyolefin PE1 Mw 3.0 × 10⁵ — 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn 8.6 — 8.6 8.6 8.6 8.6 % by mass 80 — 80 80 80 80 PE2 Mw 2.0 × 10⁶ — 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn 8 — 8 8 8 8 % by mass 20 — 20 20 20 20 1^(st) PP Mw — — — — — — Mw/Mn — — — — — — Heat of fusion(J/g) — — — — — — % by mass — — — — — — Conc. of PO Comp. % by mass 30 — 25 35 35 25 Second Polyolefin PE1 Mw — 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn — 8.6 8.6 8.6 8.6 8.6 % by mass — 69 69 69 69 49 PE2 Mw — 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn — 8 8 8 8 8 % by mass — 1 1 1 1 1 2^(nd) PP Mw — 1.10 × 10⁶  1.10 × 10⁶  0.54 × 10⁶  1.56 × 10⁶  2.67 × 10⁶  Mw/Mn — 5.0 5.0 4.7 3.2 2.6 Heat of fusion(J/g) — 114.0 114.0 94.0 78.4 99.0 % by mass — 30 30 30 30 50 Cone, of PO Comp. % by mass — 30 30 30 30 35 Production condition Extrudate Layer structure (I) (II) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) Layer thickness ratio 100 100 47/6/47 46/8/46 46/8/46 40/20/40 Total PP content % by mass 0 30 1.89 2.08 2.08 13.0 Stretching of Gel-Like sheet Temperature (° C.) 115 115 115 115 115 115 Magnification (MD × TD) 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 Stretching of dried membrane Temperature(° C.) — 127 127 127 127 127 Magnification (TD) — 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 1.6 −> 1.4 Heat setting treatment Temperature(° C.) 127 127 127 127 127 127 Time (min) 10 10 10 10 10 10 Properties Average thickness (μm) 25.5 24.9 25.1 25.4 25.1 24.6 Normalized Air Permeability 550 446 256 223 290 460 (sec/100 cm³/25 μm) Porosity % 38.4 46.1 47.0 47.8 46.7 42.6 Weight per unit area 15.6 13.3 13.2 13.1 13.2 14.0 Normalized Puncture Strength 5488 4939 5322 5107 5603 5341 (mN/25 μm) Heat shrinkage MD/TD (%) 5.5/5.0 4.3/1.8 3.4/1.4 3.2/1.2 3.6/1.5 4.2/2.1 Shut Down Temp. ° C. 132 136 132 132 132 132 Rupture Temp. ° C. 150 181 156 165 164 190 Maximum shrinkage (TMA) % 32 13 15 14 18 21 Is A ≦ 0.097P-110? No no yes yes yes no

It is noted from Table 1 that the microporous membrane of the present invention has well-balanced properties, including air permeability, pin puncture strength, shut down temperature and rupture down temperature, as well as low maximum shrinkage in the molten state. Lithium ion secondary batteries comprising the microporous membranes of the present invention have high capacity and high safety performance. As discussed in the Background, the selection of polymer type and content for microporous polymeric membranes represents a trade-off. Polymeric materials and membrane structures that conventionally provide a relatively high porosity (or high Air Permeability as characterized by a shorter time to pass a volume of air through the pores of the membrane) generally lead to a lower membrane Pin Puncture Strength, particularly when a relatively high Rupture temperature is desired, e.g., 180° C. or higher. The invention is based in part on the discovery that three-layer membranes as described above exemplified in the Examples 1 through 8 achieve an optimization of Air Permeability, Pin Puncture Strength, and Rupture temperature.

On the other hand, the microporous membranes of the Comparative Examples exhibit a poorer balance of these properties. For example, even though Comparative Examples 3, 4, and 5 have an Air Permeability satisfying the relationship A≦0.097P-110, they do not have a meltdown temperature in the range of 180° C. or higher.

The microporous membranes of the present invention have well-balanced properties and the of such microporous membrane as a battery separator provides batteries having excellent safety, heat resistance and productivity.

All patents, test procedures, and other documents cited herein, including priority documents, are fully incorporated by reference to the extent such disclosure is not inconsistent and for all jurisdictions in which such incorporation is permitted.

While the illustrative forms disclosed herein have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside herein, including all features which would be treated as equivalents thereof by those skilled in the art to which this disclosure pertains.

When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. 

1. A multi-layer microporous membrane, comprising: a first layer material comprising polyethylene and a second layer material comprising polypropylene, the polypropylene having (1) a weight-average molecular weight ≧6×10⁵, (2) a heat of fusion ≧90 J/g, and (3) an MWD in the range of 2 to 6, wherein (a) the multi-layer microporous membrane has at least a first microporous layer containing the first layer material, a third microporous layer containing the first layer material, and a second microporous layer containing the second layer material, the second microporous layer being located between the first and third microporous layers, and (b) the total amount of polypropylene in the multi-layer microporous membrane is at least 2.0 wt. % based on the total weight of the multi-layer microporous membrane.
 2. The multi-layer microporous membrane of claim 1, wherein (a) the polyethylene of the first layer material is a first polyethylene, the first polyethylene being present in the first layer material in an amount in the range of from 80 wt. % to 100 wt. % based on the weight of the first layer material, (b) the polypropylene of the second layer material is present in the second layer material in an amount in the range of from 1 wt. % to 100 wt. %, based on the weight of the second layer material, and (c) the second layer material further comprises a second polyethylene, the second polyethylene being present in the second layer material in an amount in the range of from 0 wt. % to 99 wt. %, based on the weight of the second layer material.
 3. The multi-layer microporous membrane of claim 1, wherein the first microporous layer material consists essentially of polyethylene.
 4. The multi-layer microporous membrane of claim 1, wherein the multi-layer microporous membrane is a three-layer membrane and wherein the amount of polypropylene in the second layer material is in the range of 20 wt. % to 50 wt. %.
 5. The multi-layer microporous membrane of claim 1, wherein the first layer further comprises polypropylene that is optionally the same as the polypropylene of the second layer material, the polypropylene of the first layer material being present in an amount in the range of from 0.5 wt. % to 10 wt. % based on the weight of the first layer material.
 6. The multi-layer microporous membrane of claim 1, wherein (a) the first and/or second polyethylene comprises PE1, PE2, or both PE1 and PE2, and wherein (1) the first polyethylene has a Mw in the range of 1×10⁵ to 5×10⁶; (2) the second polyethylene has an Mw ranging from 1×10⁵ to 5×10⁶; (3) PE1 is one or more of a high-density polyethylene, a medium-density polyethylene, a branched low-density polyethylene, or a linear low-density polyethylene; (4) PE1 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1; (5) PE2 has an Mw of at least 1×10⁶; (6) PE2 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1; (7) the amount of the PE1 in the first microporous layer material is in the range of from 50 wt. % to 100 wt. %, based on the weight of the first microporous layer material; (8) the amount of the PE2 in the first microporous layer material is in the range of from 0 wt. % to 50 wt. %, based on the weight of the first microporous layer material; (9) the amount of the PE1 in the second microporous layer material is in the range of from 40 wt. % to 60 wt. %, based on the weight of the second microporous layer material; (10) the amount of the PE2 in the second microporous layer material is in the range of from 0 wt. % to 50 wt. %, based on the weight of the second microporous layer material; (11) the first and/or second polyethylene has Mw/Mn in the range of 5 to 300; and (b) the second polypropylene has at least one of: (1) an Mw/Mn in the range of 2 to 6; (2) an MW in the range of 9×10⁵ to 2×10⁶; and (3) a heat of fusion is 100 J/g or more.
 7. The multi-layer microporous membrane of claim 1, wherein the second microporous layer has a thickness in the range of about 4.6% to about 50% of the total thickness of the multi-layer microporous membrane.
 8. The multi-layer microporous membrane of claim 6, wherein the first microporous layer material further comprises polypropylene.
 9. The multi-layer microporous membrane of claim 6, wherein PE1 is high-density polyethylene and the PE2 is ultra-high molecular weight polyethylene.
 10. The multi-layer membrane of claim 9, wherein the first and/or second polyethylene comprises 8 wt. % or less of PE2 and 92 wt. % or more of PE1.
 11. A method for producing a microporous membrane, comprising, (1) combining a polyethylene resin and a first diluent, (2) combining a polypropylene resin, and a second diluent; the polypropylene resin having the polypropylene having (a) a weight-average molecular weight of 6×10⁵ or more, (b) a heat of fusion of 90 J/g or more, and (c) an MWD in the range of 2 to 6; (3) extruding at least a portion of the combined first polyethylene resin and first diluent, and extruding at least a portion of the combined first polypropylene resin and second diluent to produce a multi-layer extrudate which comprises a first layer comprising the combined polyethylene and first diluent, a second layer comprising the polypropylene and the second diluent, and a third layer comprising the combined polyethylene and first diluent, wherein the second layer is located between the first and third layers and wherein the polypropylene is present in the extrudate in an amount ≧2.0 wt. % based on the weight of polymer in the extrudate; and then (4) cooling the multi-layer extrudate to form a multi-layer sheet, (5) removing at least a portion of the first and second diluents from the multi-layer sheet to form a diluent-removed sheet, and (6) removing at least a portion of any volatile species from the sheet to form the microporous membrane.
 12. The method of claim 11, wherein the combined polyethylene and first diluent is a first polyolefin solution, wherein the combined polypropylene and second diluent is a second polyolefin solution, and wherein the polyethylene resin is present in the first polyolefin solution in an amount in the range of from about 0.5 wt. % to about 75 wt. % based on the total weight of polyolefin in the first polyolefin solution, the first polyolefin solution comprises polypropylene resin, the polypropylene resin being present in the first polyolefin solution in an amount in the range of from about 0 wt. % to about 10 wt. % based on the total weight of polyolefin in the first polyolefin solution, the second polyolefin solution comprises polyethylene resin, the polyethylene resin being present in the second polyolefin solution in an amount in the range of from about 40 wt. % to about 90 wt. % based on the total weight of polyolefin in the second polyolefin solution, the polypropylene resin is present in the second polyolefin solution in an amount in the range of from about 10 wt. % to about 60 wt. % based on the total weight of polyolefin in the second polyolefin solution; the first diluent is present in the first polyolefin solution in an amount in the range of from about 25 wt. % to about 99 wt. % based on the weight of the first polyolefin solution; and the second diluent is present in the second polyolefin solution in an amount in the range of from about 25 wt. % to about 99 wt. % based on the weight of the second polyolefin solution.
 13. The method of claim 12 wherein (a) the polyethylene of the first, second, and third layer independently comprise a PE1, a PE2, or both PE1 and PE2, wherein (1) the first polyethylene has a Mw in the range of 1×10⁵ to 5×10⁶; (2) the second polyethylene has an Mw ranging from 1×10⁵ to 5×10⁶; (3) PE1 is one or more of a high-density polyethylene, a medium-density polyethylene, a branched low-density polyethylene, or a linear low-density polyethylene; (4) PE1 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1; (5) PE2 has an Mw of at least 1×10⁶; (6) PE2 is at least one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene and a comonomer selected from propylene, butene-1, hexene-1; (7) the amount of the PE1 in the first microporous layer material is in the range of from 50 wt. % to 100 wt. %, based on the weight of the first microporous layer material; (8) the amount of the PE2 in the first microporous layer material is in the range of from 0 wt. % to 50 wt. %, based on the weight of the first microporous layer material; (9) the amount of the PE1 in the second microporous layer material is in the range of from 40 wt. % to 60 wt. %, based on the weight of the second microporous layer material; (10) the amount of the PE2 in the second microporous layer material is in the range of from 0 wt. % to 50 wt. %, based on the weight of the second microporous layer material; (11) the first and/or second polyethylene has Mw/Mn in the range of 5 to 300; and (b) the second polypropylene has at least one of: (1) an Mw/Mn in the range of 2 to 6; (2) an MW in the range of 9×10⁵ to 2×10⁶; and (3) a heat of fusion is 100 J/g or more. (b) the polypropylene has at least one characteristic selected from: (1) a weight-average molecular weight of 9×10⁵ to 2×10⁶, (2) a heat of fusion of 100 J/g or more, and (3) a molecular weight distribution (Mw/Mn) in the range of 2 to
 6. 14. A battery comprising an anode, a cathode, an electrolyte, and the multi-layer microporous membrane of claim 1, wherein the multi-layer microporous membrane separates at least the anode from the cathode.
 15. The battery of claim 14, wherein the electrolyte contains lithium ions and the battery is a secondary battery.
 16. The battery of claim 14 used as a source or sink of electric charge.
 17. A microporous membrane having Rupture temperature 180° C. or higher and comprising polypropylene wherein the total amount polypropylene in the membrane is ≧2 wt. %, based on the total weight of the membrane.
 18. The microporous membrane of claim 17, wherein the total amount polypropylene in the membrane having (a) an Mw≧6×10⁵, (b) an MWD in the range of 2 to 6, and (c) a heat of fusion of 90 J/g or higher is ≧2 wt. %, based on the total weight of the membrane.
 19. The microporous membrane of claim 17 wherein the membrane has Normalized Air Permeability satisfying the relationship A≦(M*P)−I where A is the microporous membrane's Normalized Air Permeability thickness, P is the microporous membrane's Normalized Pin Puncture Strength, M is a slope in the range of about 0.09 to about 0.1, or about 0.95 to about 0.99, and I is ≧100.
 20. A battery comprising the separator of claim
 19. 21. A battery separator made by the process of claim
 11. 